The present invention relates to nozzle-flapper pilot valves used in pilot pressure control applications such as in the pilot stages of pressure controlled servovalves and, more particularly, to an arrangement wherein the nozzles responsive to flapper position are of sufficient size such that the pressure within the nozzles provides a feedback torque on the flapper of the pilot valve to substantially balance the torque applied to the flapper by the torque motor of the pilot valve.
A servovalve is one application for the present invention and, therefore, the present invention will be described in connection with servovalves. Servovalves are typically two-stage devices which respond to an input electrical signal for providing a differential pressure output utilized by a hydraulic motor for performing work. For example, such a servovalve is comprised of a pilot stage and a boost stage. The pilot stage is typically a valve having a magnetic motor which responds to an input electrical signal for positioning a flapper between a pair of nozzles. The position of the flapper between the nozzles controls the pressure within the nozzles in order to provide a controllable differential output pressure from the pilot stage.
The boost valve stage is an amplifying stage which has a spool the position of which is controlled by the differential output pressure from the pilot stage. The spool is used to control the output differential pressure from the boost valve stage. The boost valve stage differential output pressure is then used for positioning a hydraulic motor.
The magnetic motor of the pilot stage section exerts on the flapper both a magnetic spring rate as a result of the magnetic structure used in the motor and also a mechanical spring rate as a result of the centering springs used to center the flapper when the motor is deenergized. Prior art pilot stages thus had a substantial spring rate acting upon the flapper such that a significant percentage of the torque developed by the magnetic motor of the pilot stage was used in counteracting the spring rate leaving the remaining torque for positioning the flapper between the nozzles of the pilot stage. The position of the flapper in such a valve is proportional to the current input to the magnetic motor such that the differential pressure versus input current response of the pilot stage changes as the supply pressure changes.
A valve in which the differential output pressure is truly proportional to the input current such that changes in supply pressure have no appreciable affect upon the pressure versus current response of the valve is more desirable. This result can be obtained by reducing the resultant spring rate between the magnetic and mechanical spring rates applied to the flapper and increasing the pilot nozzle size so that the force applied by the nozzle pressure to the flapper is substantially greater than the resultant spring rate acting on the flapper. The feedback torque acting on the flapper from the differential nozzle pressure is sufficient, therefore, to balance the torque produced by the magnetic motor. Thus, if supply pressure changes, the flapper assumes a position to maintain a substantially constant differential output pressure across the output of the pilot stage.